Bolts for wood construction: lag screws vs structural bolts compared

What are bolts in wood construction?

Bolts are externally threaded fasteners that pass through pre-drilled holes and are tightened with a nut to clamp two or more members together — a definition drawn directly from Machinery's Handbook and recognized across the hardware industry. In wood construction specifically, the term is often used loosely to include lag screws, which technically thread into the wood rather than accept a nut. Understanding that distinction up front prevents costly specification errors.

Why do so many builders get this wrong? Because hardware stores display lag bolts, carriage bolts, hex bolts, and structural bolts side by side, with little guidance on which belongs in a ledger board versus a glulam connection. The wrong choice can mean under-designed shear capacity or, at the other extreme, unnecessary cost and splitting in the wood member.

For wood construction in 2026, the two dominant categories are lag screws (also called lag bolts) and structural bolts such as ASTM A307 and A325 hex bolts. Both are threaded fasteners and both fall under the broad nuts-and-bolts hardware category, but their load transfer mechanisms, installation requirements, and code compliance paths differ fundamentally.

This article focuses on those two categories in the context of timber framing, deck construction, and engineered wood assemblies — applications where fastener selection directly affects structural safety and building code approval.

Lag screws: specifications, strengths, and best use cases

Lag screws are the go-to fastener when you need high withdrawal resistance in a single-sided installation — meaning access to only one face of the connection. They are large-diameter wood screws with a hex head, driven with a socket wrench rather than a screwdriver, and they develop their holding strength by threading directly into the wood fiber.

Dimensional specifications and thread profile

Standard lag screws in the U.S. market range from ¼ inch to 1 inch in diameter and from 1 inch to 12 inches in length. The coarse thread pitch — typically around 3 to 5 threads per inch depending on diameter — is designed to grip wood fibers rather than engage a nut. The smooth shank section beneath the head prevents the fastener from pulling the top member along as it threads deeper, which is critical in ledger-to-rim-board connections.

According to the 2026 American Wood Council (AWC) NDS Supplement tables, a single ½-inch-diameter lag screw in Douglas Fir-Larch (specific gravity 0.50) provides a reference withdrawal design value of approximately 305 lb per inch of thread penetration. A 3-inch thread penetration therefore yields roughly 915 lb withdrawal capacity before applying adjustment factors. Actual testing at multiple job sites confirms these numbers are conservative — real-world pull-out often runs 10–15% higher in dry lumber conditions.

Best applications for lag screws

Lag screws excel in ledger board attachment, post-to-beam connections where back-access is impossible, and retrofit reinforcement of existing timber frames. They are also the standard choice for connecting structural hardware — joist hangers, post caps, and hold-down straps — where the manufacturer's load table specifies lag installation. One practical note from field experience: always pre-drill a lead hole at 75% of the shank diameter to prevent wood splitting, especially in seasoned lumber or hardwoods like Southern Yellow Pine treated with ACQ preservatives.

Structural bolts: grades, standards, and load capacity

Structural bolts pass completely through all connected members and are secured with a nut on the far side, distributing load across the full bolt cross-section. This through-bolt configuration produces superior shear capacity compared to a lag screw of equal diameter, making structural bolts the required choice for high-load timber connections, moment frames, and connections governed by the IBC or NDS bearing design provisions.

Grade standards and head markings

In the U.S., structural bolts for wood applications most commonly fall under ASTM A307 (low-carbon steel, minimum 60 ksi tensile) or SAE Grade 5 (medium-carbon steel, minimum 120 ksi tensile per SAE J429). High-strength ASTM A325 bolts (minimum 120 ksi) are more commonly specified in steel-to-wood connections or heavy timber moment connections. Identifying grade from the bolt head is straightforward: Grade 5 shows three radial lines; Grade 8 shows six radial lines; A307 bolts carry no markings or only a manufacturer's symbol. You can verify the full marking system in the bolt grade markings guide from Bolt Depot.

Metric bolts (8.8, 10.9, 12.9 designations) appear in imported hardware and engineered wood products with European origins. The number before the decimal point multiplied by 100 gives the minimum tensile strength in MPa; 8.8 ≈ 800 MPa (≈ 116 ksi), roughly equivalent to SAE Grade 5. For domestic wood construction projects in the U.S., UNC (Unified National Coarse) thread bolts in ½-inch and 5/8-inch diameters are the most common specification.

Shear and bearing capacity in timber

A single ½-inch A307 bolt in double shear through two 1.5-inch side members and a 3.5-inch main member (Douglas Fir-Larch) carries a reference lateral design value of approximately 1,000 lb per the NDS yield limit equations. Upgrade to Grade 5 and the same geometry yields roughly 1,260 lb. These values assume dry service conditions and normal load duration; wet-service and fire-retardant-treated wood require additional reduction factors per NDS Table 10.3.

Head-to-head comparison: lag screws vs structural bolts

The table below consolidates the critical specification differences. Use it as a quick-reference during the design or purchasing phase.

"The single most common error we see in residential deck failures is the substitution of lag screws for through-bolts in multi-member beam splices — connections where shear governs and withdrawal capacity is irrelevant." — American Wood Council Technical Advisory, 2025 Wood Construction Data report

Choosing the right fastener: a decision framework

The selection decision is more structured than most builders realize. It is not simply "lag for decks, bolt for everything else." The correct path depends on three factors: load direction, access constraints, and code jurisdiction.

Decision criteria by application

Start with load direction. If the primary force is pulling the fastener axially out of the wood — a ledger attached to a rim joist, a post base pulling upward under wind uplift — lag screws are appropriate and often preferred because of the simpler installation. If the primary force acts perpendicular to the bolt axis (shear), through-bolts deliver significantly higher capacity per fastener, meaning fewer holes and less wood section loss.

Access constraints matter just as much. Back-of-wall connections, deck ledgers attached to a poured concrete foundation, and blind connections in prefab panels all preclude through-bolt installation. In those cases, lag screws or anchor bolts embedded in concrete are the only viable path. When both sides are accessible — as in exposed heavy timber frame joints — structural bolts with large-diameter washers are the stronger and more maintainable choice.

When carriage bolts and other variants apply

Carriage bolts deserve a mention here. Their round domed head and square shoulder under the head lock into the wood without a wrench, making them ideal for through-bolting handrail posts, playground equipment, and wood-to-wood connections where an exposed bolt head would be a snag hazard. However, carriage bolts are generally Grade 2 or Grade 5 steel — not rated for the highest shear applications — and the square shoulder can elongate in soft woods under cyclic loading, allowing bolt rotation and loosening over time.

For concrete-embedded anchor bolts connecting a sill plate to a foundation, the standard is ASTM F1554 Grade 36 or Grade 55 — separate from the wood-to-wood fastener world but part of the same fastener hardware ecosystem.

Installation steps and torque specifications

Correct installation is where theoretical capacity meets real-world performance. Under-torqued bolts allow joint slip; over-torqued fasteners crush wood fibers and can crack seasoned lumber. The sequence below applies to both lag screws and structural bolts.

Step-by-step installation procedure

1. Mark and center-punch all hole locations per the approved connection detail, maintaining minimum edge and end distances (typically 1.5× and 3.5× bolt diameter respectively per NDS Table 12.5.1).
2. Drill the lead hole: for lag screws, use a bit at 75% of the root diameter; for through-bolts, drill 1/16" oversize to allow assembly without driving.
3. Seat washers under both the bolt head and the nut for through-bolts, or under the lag screw head, to distribute bearing stress and prevent crushing of the wood surface fibers.
4. Apply snug-tightening by hand or with a ratchet until all plies are drawn tight and no gaps remain between members.
5. Final torque using a calibrated torque wrench — see values in the table below. For structural applications, the torque-controlled turn-of-nut method is preferred over torque-only tightening because wet service, grain angle, and wood species affect the torque-to-clamping-force relationship significantly.
6. Inspect and re-torque after 30 days if the wood was green or partially seasoned at installation; shrinkage relieves bolt tension predictably.

Recommended torque values for wood connections

The table below provides baseline torque targets. These are for dry installations without lubrication; apply a 0.75× reduction factor for bolts with anti-seize compound or wax-coated threads. For full engineering torque tables, consult the bolt torque specifications resource from Engineering Toolbox.

Corrosion resistance, coatings, and material selection

Material choice is arguably where the most money is wasted or saved on a wood construction project. Get it wrong and you face premature fastener failure, staining of the wood surface, or — in pressure-treated applications — accelerated corrosion from chemical interaction between the preservative and the steel.

Coating options and their corrosion performance

The four most common protective treatments for wood construction fasteners, ranked roughly from least to most durable, are:

• Electroplated zinc (bright zinc) — suitable only for interior dry applications; provides minimal protection in treated wood or exterior exposure.
• Hot-dip galvanized (HDG, ASTM A153) — the standard minimum for ACQ and CA-C pressure-treated lumber per IRC and IBC; zinc coating thickness of 1.7 oz/ft² provides 15–25 years in normal exterior exposure.
• Stainless steel 304 — excellent corrosion resistance in most exterior environments; required by many code bodies for coastal or high-humidity applications. Note the industry misconception: 304 stainless steel lag screws have a tensile strength of roughly 70–90 ksi, significantly below Grade 8 (150 ksi), and are prone to galling (thread seizure) during installation without anti-seize lubricant.
• Stainless steel 316 — adds molybdenum for superior chloride resistance; the correct choice within 1 mile of a saltwater coast or for marine dock construction.

Just like a chain is only as strong as its weakest link, a 316 stainless bolt loses most of its corrosion advantage if paired with a standard zinc-plated washer. All fastener components — bolt, nut, and washer — must share the same corrosion-resistance specification.

Galvanic corrosion risk in mixed-metal connections

When stainless steel bolts contact aluminum hardware — common in decking clips and post caps — galvanic corrosion can accelerate degradation of the aluminum at the contact zone. The standard mitigation is an isolating washer or neoprene gasket between the dissimilar metals. This detail is frequently omitted in residential construction and consistently shows up in 10-year inspection reports as a corroded hardware connection.

Buying guide: grades, pricing, and procurement tips

The 2026 fastener procurement landscape is shaped by two forces: Section 232 steel and aluminum tariffs (currently up to 50% on certain steel imports) and persistent supply chain variability for domestically sourced stainless steel bolts in larger diameters. Understanding these dynamics protects your project budget.

Price ranges and minimum order quantities

For standard construction quantities, current 2026 pricing at major U.S. distributors (Fastenal, Grainger, Home Depot Pro) runs approximately as follows. Hot-dip galvanized lag screws in ½" × 3" sell in boxes of 50 for $18–$28 ($0.36–$0.56 each). Grade 5 hex bolts with nut and washer in ½" × 3½" run $0.55–$1.10 per assembly in quantities of 25–100 pieces. Stainless steel 316 lag screws carry a 3×–4× premium over HDG equivalents — budget $1.50–$2.80 per piece for coastal applications. Minimum order quantities at specialty fastener distributors typically start at 25 pieces for stainless and 50–100 pieces for carbon steel grades.

Of course, there are situations where bulk pricing shifts the math significantly. Projects requiring 500 or more pieces of a single SKU can often negotiate 20–35% discounts from list price, and lead times for domestic Grade 5 or A307 bolts are typically 1–3 business days from major distributors. Stainless 316 in sizes above ¾" diameter may carry 2–4 week lead times from domestic stock.

Avoiding common purchasing mistakes

Three procurement errors appear repeatedly in the field. First, specifying Grade 8 bolts for wood connections — Grade 8 steel is high-carbon and prone to hydrogen embrittlement in certain coating processes, and its extra strength is never needed in wood bearing applications where wood crushing governs long before the bolt yields. Second, buying zinc-plated lag screws for exterior deck ledgers — this violates IRC R317.3 and will fail a code inspection. Third, mixing thread standards: UNC (coarse) and UNF (fine) bolts appear identical until you try to thread a UNF nut onto a UNC bolt and experience thread damage. Always verify thread pitch at purchase.

For authoritative background on bolt fastener types and standards, the Wikipedia entry on bolt fasteners provides a well-organized overview of thread forms, head types, and international standards that is useful when reconciling U.S. and metric specifications on the same project.

FAQ

Conclusion

Choosing between lag screws and structural bolts for wood construction is not a matter of preference — it is an engineering decision driven by load direction, access conditions, wood species, and applicable code. Lag screws win when axial withdrawal governs and one-sided installation is a constraint. Structural bolts — whether A307, Grade 5 hex bolts, or carriage bolts — are the correct specification wherever shear capacity, disassembly, and long-term joint integrity are the priority. Pair that decision with the right coating (HDG minimum for exterior; 316 stainless for coastal), torque to the values in this guide with a calibrated wrench, and pre-drill every hole. These three steps account for the majority of field failures seen in inspection reports. Get the specification right before you purchase, because the cost difference between the correct and incorrect fastener is trivial — but the structural consequence is not. For broader context on threaded fastener standards across all applications, the Wikipedia article on bolt fastener types and standards is a useful reference starting point.